1. Field of the Invention
The invention relates to a method of marine seismic data gathering, wherein a plurality of seismic signal transmitters and a receiver cable are towed behind an oceangoing vessel.
2. Description of the Prior Art
In geophysical investigations of the sea bed, it is necessary to procure data characteristic of the individual earth formations. This is done by sending out and recording seismic signals, which are emitted from an energy source as pressure pulses. The pressure pulses propagate downwardly to the geological stratifications, and some are reflected back and recorded. For each firing, the acoustic cable registers a reflection pattern. By continuously measuring this reflection pattern, one obtains a picture of the geological formation with strata, faults, etc. The energy source normally used in seismic explorations is the so-called air gun; such guns are towed behind a vessel and the reflected pulses are registered by means of an acoustic cable. The air guns work by releasing a certain volume of air which oscillates as a slowly decaying wave, the period of oscillation being dependent on the volume of air released.
In pulse reflection, however, it is advantageous if the wave can be given as "peaked" a shape as possible, i.e., so as to minimize pulse reverberations and thus obtain as pure a transmitter signal as possible. For this purpose, a plurality of mutually tuned air guns generating pulses simultaneously have been employed, being so tuned that pulse reverberations are compensated/counteracted while the first half-period of oscillation is reinforced. The guns are placed within a restricted area (in an array), and they may be considered as one transmitter which via the cooperation of the guns provides a sharp or high-peak pulse. Such transmitter sources may be termed "point sources".
These known point sources will give satisfactory signals, but since they are "nondirectional" transmitters it is difficult to determine the point of origin of the reflection, i.e., the exact position of the place being examined. In addition, there is the problem of background noise, i.e., disturbing reflections from the surface of the sea bed, reflections from a location lying in another direction in cases where a plurality of transmitters are being used, etc.
A directional transmitter would be desirable, i.e., a transmitter which provides a dominance in a predetermined direction of reflection. A known solution to this problem is to place several of the above-mentioned "point sources" in a row or array, whereby the array provides directivity in the vertical direction for the signals/reflections. Such a system is described in Norwegian Pat. No. 138,922 and corresponding British Pat. No. 1417948, dated Dec. 17, 1975, and is termed "Superlong Airgun Array".
When this system is used in practice, two to four arrays of point sources are towed behind a ship, with an acoustic cable provided with hydrophones positioned centrally between the rows. The signals which are transmitted will have directivity in the vertical direction and provide a good picture of the region being investigated.
This system, however, also has weaknesses, arising among other things because the sea bed of course is not level. Despite the fact that the concentration of the waves by means of the point sources provides as pure peaks as possible, and despite the directivity in the vertical direction one obtains, it is not possible, when the region being investigated is irregular, to avoid dispersion of the reflection and associated background noise when irregularities, a fall-off of the sea bed, etc., are encountered, and as a result there is a factor of uncertainty in connection with the information obtained.